ANALYSIS AND DESIGN OF METASURFACES AND BODY-WORN ANTENNAS

Restricted (Penn State Only)
Author:
Martinez, Idellyse
Graduate Program:
Electrical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
November 16, 2018
Committee Members:
  • Douglas H. Werner, Dissertation Advisor
  • Pingjuan L. Werner, Committee Chair
  • Julio Urbina, Committee Member
  • Ramakrishnan Rajagopalan, Committee Member
  • Ramakrishnan Rajagopalan, Outside Member
Keywords:
  • Metasurfaces
  • Electromagnetic Metamaterials
  • Reconfigurable Absorber
  • Beam Steering
  • High Impedance Surface
  • Phase Gradient
  • Electromagnetic Band gap
  • Surface-waves
  • Reduction of Coupling
  • On-body Antennas
  • SAR
  • Textile Antennas
  • Wearable Antennas
Abstract:
Over the past couple of decades, there has been considerable research effort devoted to the study and development of electromagnetic configurations that utilize metasurfaces. An important subset of these metasurfaces is comprised of a subwavelength periodic array of metallic elements printed on a thin dielectric substrate that is backed by a metal ground plane or the so-called mushroom-type structure. This type of structure exhibits several novel electromagnetic properties such as in-phase reflection for incident waves which makes it practical for use as a narrowband artificial magnetic conductor (AMC). Additionally, it behaves as an electromagnetic band gap (EBG) structure in that it generates a forbidden band of frequencies where surface-waves cannot propagate. These two types of behaviors directly depend on the geometrical characteristics of the unit cell as well as the constitutive material’s properties. The first part of this dissertation presents a design methodology for the synthesis of absorbers, phase gradient metasurfaces, and EBG structures. The methodology used herein is based on multiport network representation of the loaded mushroom-type metasurfaces. The analysis reveals that by incorporating a port-reduction technique in conjunction with a powerful global optimization algorithm like the Covariance Matrix Adaptation Evolutionary Strategy (CMA-ES), the loaded metasurface can be tuned to have the desired response (e.g., reduction of RCS, reduction of coupling). The applicability of this methodology is demonstrated with several representative examples and different functionalities. Foremost, the mushroom-type unit cell is applied to the AMC case where the reflection of the incident wave is manipulated to develop an absorber structure. The mushroom-type metasurface is used because it offers simplicity in fabrication and ease of tunability by incorporating multi-circuit loadings; its frequency of operation can be modified or tuned by appropriately choosing the lumped elements (e.g., capacitor and resistor values) needed to achieve the desired absorption response. Moreover, this type of metasurface is also used to create a reconfigurable phase-gradient structure. Again, by capacitively loading the unit cells, the reflection phase can be shifted, allowing the metasurface to steer the incoming wave to the desired direction, behaving as a phase gradient absorber. A formula based on antenna array theory is employed to determine the required phase shift needed to steer the main beam. Unlike previous static design approaches that yield fixed beam patterns, the reconfigurable aspect of the metasurface allows the dynamic control over the placement of the RCS peaks and/or nulls. Moreover, it is shown that by combining resistors with the tunable capacitive elements, additional control over the RCS properties can be attained. In addition, a new design methodology for optimizing mushroom-type EBG structures to reduce the mutual coupling between antenna arrays is proposed. Once again, by capacitively loading the unit cells, not only is the size of the EBG reduced, but also the integrated antenna-EBG system can be efficiently optimized as a single integrated structure. By employing the port-substitution method in conjunction with the global CMA-ES, the otherwise lengthy optimization process is reduced to a fraction of the time. In fact, employing the port-substitution method allows the optimization process to be thirty-five times faster than optimizing using a full-wave simulation solver. Both simulation and measurement results show improvement of mutual coupling and return loss between two patch antennas. The advantage of this methodology over the traditional approach of designing the EBG in isolation without considering the antennas interaction is explained in detail. Additionally, the versatility of the new design methodology is demonstrated by applying it to several unique cases, including dual-band mutual coupling reduction. The second part of this dissertation presents research on novel body-worn antennas. Body-worn antennas are a critical component in wearable technologies. Wearable antenna technology has rapidly evolved in the last decades, leading to a future where e-textiles will be integrated into everyday garments. However, the wearable antenna technology has several challenges; the antennas must be lightweight and low-profile, must have a robust performance due to bending and/or stretching, and must satisfy the FCC requirements for the specific absorption rate, among other qualities. All of these factors must be fulfilled while working in close proximity to the human body. This dissertation will address some of these challenges regarding wearable antennas, in particular designs that incorporate textile materials. Although textile materials are desirable for ease of integration into a wearable garment, the material’s variability makes it difficult to develop robust antenna designs. In the second part of this thesis, a low-profile wideband linearly polarized proximity-fed antenna is introduced for a robust off-body communication link. This antenna design is compared to a textile microstrip-fed linearly polarized patch antenna, and the results of this comparison reveal that the proposed antenna has much more stable performance due to the dielectric’s constant variability as well as broader bandwidth. The bandwidth limitation of the microstrip antenna can be a problem when considering the material’s dielectric constant variation. The antenna was fabricated using three different material systems, two textile-based antennas, and its PCB-counterpart. Simulation and measurement results show that this antenna design has robust performance both in free space and due to human body loading effects. Overall, this simple yet effective wearable antenna offers a compact, lightweight, low-profile, flexible, and robust solution, which makes it a strong candidate for integration into wearable garments. Due to the textile material’s variability, it is difficult to realize multifunctional designs. For this reason, the majority of the textile antennas are linearly polarized, which means that the communication link would be sensitive to human body motion. Therefore, a robust metasurface-based circularly polarized wearable textile antenna for off-body communications is proposed. The CP metasurface antenna possesses a stable impedance matching, wide axial ratio bandwidth, and exceptional gain performance even under the expected range of possible textile material variations. As such, the proposed antenna represents an ideal candidate for integration into a textile system.